Modern gas turbine engines, and more specifically turbofans for use in aviation, provide power by compressing air using a compressor, adding fuel to this compressed air, combusting this mixture such that it expands through the blades of a turbine and exhausting the produced gases. The turbine consists of a disc, rotating about the central shaft of the engine, and a plurality of blades extending radially out of the disc towards the engine casing of the engine. Expansion of the combustion gases through the turbine causes its blades to rotate at high speed and the turbine, in turn, drives the compressor.
The distance between the tips of the blades and the inner surface of the turbine casing is known as the tip clearance. It is desirable for the tips of the turbine blades to rotate as close to the engine casing without rubbing as possible because as the tip clearance increases, a portion of the expanded gas flow will pass through the tip clearance the efficiency of the turbine decreases as. This is known as over-tip leakage. The efficiency of the turbine, which partially depends upon tip clearance, directly affects the specific fuel consumption (SFC) of the engine. Accordingly, as tip clearance increases, SFC also rises.
As the disc and the blades of the turbine rotate, centrifugal and thermal loads cause the disc and blades to extend in the radial direction. The turbine casing also expands as it is heated but there is typically a mismatch in radial expansion between the disc/blades and the casing. Specifically, the blades will normally expand radially more quickly than the housing, reducing the tip clearance and potentially leading to “rubbing” as the tips of blade come into contact with the interior of the casing of the turbine. Over time in use, the casing heats up and expands away from the blade tip, increasing the tip clearance. This may result in a tip clearance at stabilised cruise conditions that is larger than desired resulting in poor efficiency.
Conventionally, tip clearances are set when the engine is cold to allow for radial extension of the turbine disc and blades due to centrifugal and thermal loads, to prevent rubbing. This means that there is initially a large tip clearance, such that the engine is relatively inefficient. When the engine is running, the blades will eventually extend radially to close this clearance, making the engine run more efficiently. Over a longer period of time, however, the temperature of the turbine casing will rise and the casing will expand radially, which will again increase the tip clearance.
Currently technology to overcome this problem uses a cooling duct extending around the circumferential outboard surface of the turbine casing, into which bypass air is fed to impinge on and cool the turbine casing during stabilised cruise conditions. By cooling the casing in this way the radial expansion of the casing is lessened and a smaller tip clearance is maintained.
However, this current approach to controlling tip clearance lacks the responsiveness required to maintain an appropriate tip clearance during transient parts of the flight profile, such as during take-off or step climb. Under these conditions, the disc and blades expand radially much more quickly than the casing, with the risk of rubs between the blade tip and the casing unless the tip clearance is set larger than would otherwise be desirable when the engine is cold.